1. Field of the Invention
The present invention generally relates to ink jet heads and, more particularly, to an ink jet head for use in an ink jet printer constructed such that a piezoelectric plate member is deformed to have a dome configuration so as to cause the volume of a pressure chamber to vary and cause the ink in the pressure chamber to be ejected.
There is a demand for an ink jet printer to have an improved printing resolution and printing speed. In order to meet such a demand, the operation for deforming the piezoelectric plate member to bulge into a dome configuration should be performed efficiently.
2. Description of the Related Art
FIG. 1A is a partial front sectional view of an ink jet head 10 showing a pressure chamber 11 of a plurality of pressure chambers provided in the ink jet head 10; FIG. 1B is a partial enlarged view of the ink jet head 10 of FIG. 1A; FIG. 1C is a lateral sectional view of the ink jet head 10 of FIG. 1A; and FIG. 1D is a bottom view of the ink jet head 10 of FIG. 1A.
In the ink jet head 10, the volume of the pressure chamber 11 containing the ink is cyclically reduced and restored by cyclically deforming an oscillating plate 13 (for example, a stainless plate of a thickness of 20 .mu.m) and returning it to an original state. When the volume of the pressure chamber 11 is reduced, ink particles 15 (approximately 80 pl) are ejected from a nozzle 14. When the volume of the pressure chamber 11 is restored to an original level, the ink inside a common passage 16 is supplied to the pressure chamber 11 via an ink supplying passage 17.
The ink jet head 10 further comprises barriers 20 and 21, a top plate 22, a nozzle plate 23 and a piezoelectric plate member 25.
The piezoelectric plate member 25 has a common electrode 26 on the upper surface thereof and an individual electrode 27 on the lower surface thereof. The piezoelectric plate member 25 is attached to the lower surface of the oscillating plate 13 via a adhesive layer 28. The oscillating plate 13 and the piezoelectric plate member 25 are integral with each other so as to form a flat bimorph element 29.
The pressure chamber 11 generally has a rectangular solid configuration. The oscillating plate 13 constitutes a bottom plate for the pressure chamber 11 and has all four sides thereof adhesively attached to the chamber 11.
As shown in FIG. 1D, the common electrode 26 and the individual electrode 27 both have a rectangular configuration of a size that corresponds to the size of the bottom of the pressure chamber 11.
A voltage is applied across the individual electrode 27 and the common electrode 26 so that the piezoelectric plate member 25 is polarized in a direction of the thickness thereof.
The piezoelectric plate member 25 has a piezoelectric constant d33 in a direction of the polarization (that is, in the direction of the thickness) of the piezoelectric plate member 25, and a piezoelectric constant d31 in a direction perpendicular to the direction of polarization (that is, in the direction along the plane). Distortion caused by expansion and contraction of the piezoelectric plate member 25 is determined by the intensity of the electric field exerted on the piezoelectric plate member 25.times.piezoelectric constant.
Generally, the piezoelectric constant d33 is larger than the piezoelectric constant d31 such that d33 may be twice as large as d31.
The ink jet head 10 is capable of printing at a resolution of 300 dpi. A distance a between the adjacent nozzles 14 (equal to a distance between the barriers 20 and 21) is 0.339 mm. An area S1 of a bottom of the pressure chamber 11 is a.times.b.
A description will now be given of bulging distortion the bimorph element 29.
When a switch 40 shown in FIG. 1A is closed, a voltage from a power source 41 is applied to the individual electrode 27 so that an electric field 42 indicated by the arrows is produced between the individual electrode 27 and the common electrode 26 of the piezoelectric plate member 25 in the direction of the thickness of the bimorph 29. As a result, the piezoelectric plate member 25 expands in a direction indicated by an arrow 43 and contracts in a direction indicated by an arrow 44 in a plane direction perpendicular to the direction of the electric field 42. Due to the contraction of the piezoelectric plate member 25 in the plane direction, the bimorph element 29 bulges into a dome configuration, as indicated by the alternate long and two short dashes line. When the switch 40 is grounded, the electric field 42 is removed so that the distortion due to the contraction and the expansion is gone. The bimorph element 29 is restored to a flat state due to elasticity of the oscillating plate 13.
Since d31&lt;d33, the efficiency of contraction of the piezoelectric plate member 25 in the direction along the plane is lower than the corresponding efficiency in the direction of the polarization. The efficiency with which the bimorph element 29 is bulged into a dome configuration is not satisfactory.
As a result, the ink particles 15 ejected from the nozzle 14 may be insufficient in volume for proper printing, if the pressure chamber 11 is relatively small. For this reason, reduction of the size of the pressure chamber 11 is difficult. Thus, the goals of improving the printing resolution and printing speed are difficult to achieve.